U.S. Pat. No. 6,852,172 discloses a milking plant for milking animals, such as cows, wherein milk collecting means, preferably in the form of teat cups, are attached to the teat of the animal after which milk is taken from the animal by use of vacuum. The milk is transported by a milk line system to milk storage means comprising a cooling tank. For cleaning purposes the milk line system from each milk collecting means can be shut off from the milk tank by valve means in order for separate cleaning of the milk line system. The invention is characterized by preventing any possible leakage of detergent from a cleaning fluid to the milk that has been collected in the milk tank, by providing a pressure difference between the two fluids. The pressure difference between the fluids is preferably achieved by connecting the cleaning fluid to the vacuum supply of the milking plant.
A problem of such approach is that one of the fluid systems has always to have a lower internal pressure than the other fluid system, that is, in the above application the cleaning fluid system is connected to vacuum whereas the milk tank system is kept at a higher pressure. Thus, one is restricted to use vacuum based cleaning techniques. Further, if the vacuum fails, the pressure will increase in the cleaning fluid system possibly to a hazardous level where there is risk of leaking cleaning fluid into the milk tank system.
Devices based on a so-called block-bleed-block valve assembly have been provided to remedy the above mentioned problem and to fulfill regulations concerning the handling of fluids for human consumption. A block-bleed-block valve assembly comprises two valve means serially connected between two fluid systems that should not be mixed. The cavity formed between the valve means and the two fluid systems are held at pressures so that the pressure in the cavity is always lower than the pressure in each of the fluid systems. Preferably, the cavity is held at underpressure. The cavity is typically connected to a drain for draining fluid leaking from the fluid systems into the cavity.
A problem of such block-bleed-block valve assembly approach is that the cavity and drain are difficult to clean. Further, if the drain is open to the atmosphere a contamination risk is achieved. Bacteria may enter the drainage and contaminate the cavity and the fluid systems. Another problem is that the operation of the block-bleed-block valve assembly may be difficult to monitor, in particular when the cavity already contains fluid from a least one of the two fluid systems.